1. Field of the Invention
The present invention relates in general to a resistor element, and more particularly to a resistor element which utilizes temperature dependence of an electrical resistance of an electrically resistive body, and which may be suitably used as a thermal flowmeter for measuring an amount or a flow rate or velocity of a fluid flowing in an internal combustion engine, for example.
2. Discussion of the Prior Art
There are known resistor elements of the above type having a cylindrical shape or a planar shape. FIG. 1 shows an example of cylindrical resistor element, which includes a ceramic tube 2 formed of alumina, for example, and having an outside diameter of about 0.5 mm, and a platinum thin layer 8 which is formed on an outer surface of the tube 2 and suitably patterned so as to have a desired resistance value. A pair of electrical conductors or leads 4 in the form of platinum wires having a diameter of about 0.2 mm are inserted into the opposite end portions of the ceramic tube 2, and are secured thereto by respective masses of platinum paste 6 serving as adhesives. The platinum layer 8 and leads 4 are electrically connected to each other, through the adhesive masses 6 provided at the opposite end portions of the ceramic tube 2.
Where the platinum wires are used as the leads as in the known resistor element, however, the heat generated by the resistor element tends to escape through the leads, rather than being dissipated into a fluid to be measured as intended. Accordingly, the temperature of the resistor element cannot be responsive to the amount or flow rate of the measurement fluid, resulting in reduced detecting accuracy of the element and a prolonged time required to start its detecting operation.
To solve the above problem, the leads may be formed of a material, such as 40Ni-Fe alloy, which has a lower thermal conductivity than platinum. In this case, however, the resistor element suffers from poor adhesion between the leads and the adhesive masses prepared from the platinum paste. When tensile stresses are exerted on the leads during installation of the resistor element by welding, therefore, the leads are likely to be pulled out of the adhesive masses, giving rise to clearances or cracks and accordingly reduced contact between the leads and adhesive masses. Consequently, the resistor element greatly suffers from a change in its electrical resistance. Since various thermal stresses are applied to the resistance element during its use, causing clearances or cracks between the adhesive masses and leads, the resistor element in use also suffers from a change in its electrical resistance.
To improve the adhesion between the leads and the adhesive masses prepared from the platinum paste, and to prevent oxidization of the leads during manufacture of the resistor element, it is proposed to employ a lead which consists of a wire rod or core formed of 40Ni-Fe alloy, and a platinum layer formed on the outer surface of the wire rod. While the thus formed lead is less likely to be pulled out of the adhesives, the thermal conductivity of the lead as a whole is undesirably increased because of the platinum layer, causing the same problem as encountered when the lead is formed solely of platinum.